The most natural reaction, although it's hard to say if it's better than capturing the knight.
5. b1c3  5... f8b4 A good option, but not forced. 6. g1e2 A rare move. Van Foreest is a known opening specialist, and he is usually the first to show a new idea. 6... g8e7  7. a2a3 This is the idea, now the knight can capture the bishop on c3.